The present invention relates generally to mobile satellite communication systems, and more particularly to a method and system for providing alert messaging to mobile terminals in high-attenuation propagation environments within a mobile satellite communications network.
Terrestrial communications systems continue to provide higher and higher speed multimedia (e.g., voice, data, video, images, etc.) services to end-users. Such services (e.g., Third Generation (3G) and Fourth Generation (4G) services) can also accommodate differentiated quality of service (QoS) across various applications. To facilitate this, terrestrial architectures are moving towards an end-to-end all-Internet Protocol (IP) architecture that unifies all services, including voice, over the IP bearer. In parallel, mobile satellite systems (MSS) are being designed to complement and/or coexist with terrestrial coverage depending on spectrum sharing rules and operator choice. With the advances in processing power of desktop computers, the average user has grown accustomed to sophisticated applications (e.g., streaming video, radio broadcasts, video games, etc.), which place tremendous strain on network resources. Internet services, as well as other IP services, rely on protocols and networking architectures that offer great flexibility and robustness; however, such infrastructure may be inefficient in transporting IP traffic, which can result in large user response time, particularly if the traffic has to traverse an intermediary network with a relatively large latency (e.g., a satellite network). To promote greater adoption of data communications services, the telecommunications industry, from manufacturers to service providers, has agreed at great expense and effort to develop standards for communications protocols that underlie the various services and features.
Satellite systems, however, pose unique design challenges over terrestrial systems. That is, mobile satellite systems have different attributes that make terrestrial designs either not applicable or inefficient for satellite systems. For example, satellite systems are characterized by long delays (as long as 260 ms one-way) between a user terminal device and a base station compared to the relatively shorter delays (e.g., millisecond or less) in terrestrial cellular systems—which implies that protocols on the satellite links have to be enhanced to minimize impact of long propagation delays. Additionally, satellite links typically have smaller link margins than terrestrial links for a given user-terminal power amplifier and antenna characteristics; this implies that higher spectral efficiency and power efficiency are needed in satellite links. Moreover, the satellite transmission or channel resources represent limited resources, where the deployment of additional transmission resources is significantly more expensive, difficult and time consuming as compared with terrestrial radio resources. Accordingly, the transmission resources of a satellite system must be used efficiently to maximize the available bandwidth, in order to provide the required quality of service for the extensive and diverse assortment of service applications available to the mobile user, and to maximize the number of potential users in a system without adversely affecting the quality of service.
Moreover, in mobile satellite communication systems, user terminals (UTs) (e.g., mobile terminals) typically employ a low gain omnidirectional antenna (e.g., of less than 6 dB gain). The antenna collects the transmission signal transmitted within the spot beam of an orbiting satellite, including the direct line-of-sight components of the signal and the specular ground reflection components near the terminal. The antenna also collects multipath reflection components of the direct signal from taller stationary objects such as trees, mountains, and buildings. Such reflection components can combine destructively when collected, and result in attenuation or fading of the signal. Further, more severe signal fading or attenuation may occur if the line-of-sight path between the mobile terminal and the orbiting satellite is blocked by a building or other object. This effect is called “shadowing.” Under certain circumstances, therefore, where the shadowing and reflective factors may be enhanced (e.g., when the UT is within a metal-framed building, underground or otherwise experiencing severe signal fading or attenuation), the UT might be unable to receive a paging or alert signal transmitted by a network gateway via the satellite. The user or called party thus has no way of knowing that incoming calls are being lost. Accordingly, these factors contribute to lower success rates of conventional mobile terminated calls.
To address the problems associated with shadowing and reflective factors, current mobile satellite systems employ an alerting method to provide alert messaging to a mobile terminal being called, when the UT is within a heavily shadowed area. Alerting provides a high level announcement to a UT of a mobile terminated call, which provides the user with a notification and the opportunity to move to a less heavily shadowed area to receive the incoming call. Such an alerting method, as employed by current mobile satellite systems, is described in U.S. Pat. No. 5,974,092, titled “Method and System For Mobile Alerting in A Communication System.” Current mobile communications systems, however, utilize a 6 PSK modulated signal waveform in conjunction with a conventional orthogonal sequence, exhibiting high peak to average power ratio and irregular power spectrum, which prohibits efficient power amplification. Such systems, therefore, fail to provide sufficient link margin to provide a reliable alert messaging approach under circumstances of high attenuation. Moreover, the burst structure and coding of such systems does not facilitate the use of joint sequence detection in the receiver and forces the use of hard decision based FEC decoding. The alert message burst format of such current systems is described in GMR-1 05.002 (ETSI TS 101 376-5-2): “GEO-Mobile Radio Interface Specifications; Part 5: Radio interface physical layer specifications; Sub-part 2: Multiplexing and Multiple Access; Stage 2 Service Description; GMR-1 05.002” (V-1.2.1) (hereinafter referred to as “ETSI TS 101 376-5-2”). The FEC channel coding employed in such current systems is described in GMR-1 05.003 (ETSI TS 101 376-5-3): “GEO-Mobile Radio Interface Specifications; Part 5: Radio interface physical layer specifications; Sub-part 3: Channel Coding; GMR-1 05.003” (V-1.2.1) (hereinafter referred to as “ETSI TS 101 376-5-3”). The modulated signal waveform of such current systems is further described in the European Telecommunications Standards Institute (ETSI) publication GMR-1 05.004 (ETSI TS 101 376-5-4): “GEO-Mobile Radio Interface Specifications; Part 5: Radio interface physical layer specifications; Sub-part 4: Modulation; GMR-1 05.004” (V-1.2.1) (hereinafter referred to as “ETSI TS 101 376-5-4”). Accordingly, the system and method of alert messaging in current mobile satellite communications systems fails to provide for optimal waveforms for high penetration alerting, and for receiver algorithms that facilitate joint sequence detection and soft decision decoding.
What is needed, therefore, is an approach for high penetration alerting in a mobile satellite communications system that employs an enhanced waveform design that exhibits lower peak-to-average power ratio and permits joint sequence detection. What is also needed, therefore, is and approach for high penetration alerting in a mobile satellite communications system that employs an enhanced receiver algorithm that facilitates computationally efficient joint sequence detection and soft decision decoding.